Urinary type plasminogen activator (u-PA) is present either as a single chain proenzyme form (scu-PA) with a very low plasminogen activating activity or as an activated two-chain form (tcu-PA). tcu-PA occurs as both high molecular weight (HMW tcu-PA) or low molecular weight (LMW tcu-PA) enzyme species. HMW tcu-PA is 2-3 times more potent than LMW tcu-PA in biological assays such as the clot lysis method but their amidolytic activity on the chromogenic substrate S-2444 is equivalent (1) . The International Reference Preparation (IRP) of urokinase (66/46) was established by the Expert Committee on Biological Standardisation of the WHO in 1968 (2) . Until then urokinase was expressed in casein Ploug (Leo standard) or CTA units (CTA standard). These units were related to the amount of urokinase capable to hydrolyse a certain amount of casein in a defined time. 1 CTA-U was assumed to be identical to 1 International Unit (IU). Although there was no official numerical relationship between Ploug units and IU, 1.5 IU has been accepted to be equivalent to 1 Ploug unit. The IRP 66/46 is a mixture of 66% LMW and 34% HMW forms of tcu-PA (3) .
Following the need of a HMW International Standard, a new international standard (87/594) was established in 1989 (4) .
Due to the complexity of u-PA it is difficult to determine a direct relationship between IU, CTA-U, Plough-U and nkat S-2444 .
In 1983 Friberger (5) reported that
0.34 nkatS-2444 correspond to 110 Plough-U, 160 CTA-U or about 160 IU.
In an earlier work by Paar D and Marhulm D, on urokinase purified from urine (6) , 100 CTA-U corresponded to 0.27 nkatS-2444.

1. Gaffney PJ.
   Standards in fibrinolysis-current status and future challenges. Thromb Haemost 74, 1389-1397 (1995).
2. World Health Organisation.
   Twenty First Meeting. Expert Committee on Biological Standardisation. WHO Tech Rep Ser No 413, 1968.
3. Philo RD and Gaffney PJ.
   Assay methodology for urokinase. Its use in assessing the composition of mixtures of high- and low- molecular weight urokinases. Thromb Res 21, 81-88 (1981).
4. Gaffney PJ and Heath AB.
   A collaborative study to establish a standard for high molecular weight urinary-type plasminogen activator (HMW/u-PA). Thromb Haemost 64, 398-401 (1990)
5. Friberger P et al. Synthetic peptide substrate assays and fibrinolysis and their application on automates. In: Sem Thromb and Haemost, Vol. 9, 281-300 (1983).
6. Paar D and Marhulm D. Spectrophotometric determination of urokinase in urine after gel filtration, using the chromogenic substrate S-2444. J Clin Chem Clin Biochem 18, 557-562 (1980).